Does it have to be one story, it can it be a bunch of different, shorter horror stories. (In that case, you might want to take some of the shorter PP Flash fiction pieces like Rite of Atonement.) Either way, what's the total time for the story/stories?
I had planned on using Rite of Atonement as the first story followed by Sleepytime's "Ablutions". The lyrics are very apporpriate to the story...
She folds her arms up nicely
Lays them down Upon the table
Wipes her mouth off as she
Tries to find another word for love
She hangs her skin up by the
Window looks to see that all the
Doors are open
The She sits alone and
Wonders where you've gone
I counted your lingering
I counted time between thunder
And lightning
I counted into silence, counted chickens,
Counted sheep, counted 1,2,3,4,5,6,7...
Upstairs she hears the space
Between the words you never said
She licks the tiny stone she gave herself
And swallows all her worry
Won't you leave me forever, won't you leave
For good?
I did find a few other flashes that I liked like "Secret Boxes" but most weren't quite what I was looking for.